Calibration is necessary for circuits whose behavior depends on certain parameters e.g., resistance, capacitance, inductance or time constant, τ. Circuits whose behavior may depend on resistance may include bias networks and termination cells. Circuits whose behavior may depend on time constants, τ, may include filters, equalizers, VCOs and amplifiers. On-chip variation of resistance and of τ (τ being a function variously of resistance, capacitance, and inductance) is problematic. The solution has been to make the resistance and τs adjustable. An integrated circuit chip may have one or a number of circuits whose resistance and τs are to be calibrated. One way to provide for resistance adjustment is just to make the resistance manually adjustable; another is to have a number of resistances that are programmable by an external PROM, for example, to obtain the desired value. An automatic adjustable technique uses a pair of matched current sources that each provide a monotonic current (equal or precisely ratioed) to one of an external off-chip reference resistance and an on-chip internal resistance. The internal resistance is a replica of the resistance in the circuit to be calibrated. Any difference between the voltage on the off-chip reference resistance and the voltage on the on-chip internal resistance produces an error signal which represents as well the error between the reference resistance and the resistance in the circuit to be calibrated. This is so because the internal resistance is a replica of the resistance in the circuit to be calibrated: for example, both are on the same chip made by the same process. The error signal is used by a state machine to adjust the resistance in the circuit to be calibrated. Automatic adjustment of the time constant, τ, or the implicated resistance, capacitance, or inductance in a circuit to be calibrated is more complicated. An external clock defines a period during which the frequency of an on-chip internal circuit which is a replica of the circuit to be calibrated is compared to a known, desired reference frequency. Any difference in the frequency indicates an error in the time constant τ. That error is used by a state machine to adjust the resistance, capacitance, inductance in the circuit to be calibrated to bring its τ into conformance with the desired reference τ.